1. The Field of the Invention
The present invention broadly relates to the washing of glassware in aqueous strongly alkaline solutions. In one of its more specific variants, the invention is concerned with a method of washing soiled hot end coated glassware in an inhibited aqueous caustic soda cleaning solution whereby the resulting cleaned glassware may be recycled. The invention further relates to a novel aqueous caustic soda cleaning solution and an additive composition which is especially useful in preparing and maintaining the same.
2. The Prior Art
A variety of foods and beverages such as milk, soft drinks and beer are packaged in returnable glass bottles which are refilled and recycled a number of times during their normal life. When returnable bottles are emptied by the consumer and returned to the bottler for refilling, they must be washed thoroughly to remove all soil or other residues and thereby render the bottles physically clean. It is also necessary to sterilize the bottles and remove all microbial contamination. The most suitable washing solutions in use at the present time are highly alkaline aqueous solutions of alkali metal hydroxide. Sodium hydroxide is usually lower in cost and is preferred. Other ingredients which may be present include alkali metal carbonates, orthophosphates, pyrophosphates, polyphosphates, silicates, and borates, and various surface active agents.
The surface of glassware which is washed repeatedly in the above described solutions is subject to alkali attack. This is especially pronounced in instances where returnable bottles are washed in a mechanical soaker-washer machine and contacted therein with a heated washing solution over a number of cycles. As a result, the defaced glass surfaces take on a scratched, etched or worn appearance which is often initiated by mechanical abrasion and then accentuated by alkali attack. The term "scuffing" is commonly used to describe the pitting, knicking, scratching, wear and general defacement of the surfaces of glassware containers through the combined forces of mechanical abrasion, chemical corrosion and weathering.
Glass derives its strength from an unblemished surface and any flaws therein decrease the strength to only a fraction of the original value. Scuffing reduces the strength of glass bottles and renders them more liable to breakage during handling and in the case of carbonated beverages, explosion following filling and crowning. As a result, a number of efforts have been made heretofore to reduce scuffing to a minimum.
One of the most successful methods of reducing scuffing is by hot end coating the glassware with a substantially colorless and transparent thin protective coating including at least one organic coating material which is applied to the glass surface while hot. In accordance with one variant of the hot end coating method, the surface of the glassware is sprayed with a dilute solution or aqueous emulsion of the organic coating material such as polyethylene and/or an alkali metal salt of a fatty acid at an elevated temperature which is below the annealing temperature to thereby deposit a tightly adherent abrasion resistant organic film. In accordance with one preferred variant, the glass surface is provided with a thin transparent, substantially colorless dual protective coating which has outstanding abrasion resistant properties. The coating immediately adjacent the glass surface is a layer of a metal oxide and the second layer is an organic coating material which is applied thereover and bonded thereto. In one of the most preferred variants, a first layer of tin, zirconium or titanium oxide is pyrolyzed onto the hot glass surface, and a dilute aqueous emulsion of polyethylene or a mixture of polyethylene and an alkali metal salt of a fatty acid is applied over the first layer and bonded thereto while the glass surface is still hot.
Hot end coated glassware is much stronger and abrasion resistant initially than uncoated glassware. However, hot end coated glassware does have one severe deficiency in that when it is cleaned repeatedly in conventional soaker-washers using highly alkaline cleaning solutions, there is a very objectionable discoloration and a marked loss of lubricity. Apparently at least the outer organic protective coating layer is altered and/or either partially or completely removed by the repeated washings and the washing solution is then free to attack the inner metal oxide layer when present. The discoloration is unsightly and not acceptable to the bottling industry from the esthetic standpoint. The loss of lubricity renders the glass surface subject to scuffing as was true of the uncoated glass surfaces of the prior art. A suitable method of effectively overcoming the foregoing deficiency of hot end coated glassware was not available prior to the present invention in spite of the great need therefor.